Entrance communication system which include a number discriminator and automatic dial-up of a selected conventional telephone in an apartment from an entrance telephone set via a central switching office in the public subscriber telephone network are previously known, providing voice communication and also facilitating other services, e.g. unlocking of the entrance door. Such an entrance telephone system is disclosed in U.S. Pat. No. 3,947,641 issued March 30, 1976 to A. E. Trell.
U.S. Pat. No. 4,113,986 issued Sep. 12, 1978 to Clement et al, discloses a "hybride solution" involving direct local circuits to the conventional telephone sets of each tenant, and as a result, the entire building must be wired, since nobody can be reached via the public telephone network only. This is obviously a costly solution. This disadvantage also relates to intercom systems based on the use of conventional telephone sets located with each tenant, e.g. as disclosed in U.S. Pat. No. 3,484,561 (Matthews) and U.S. Pat. No. 3,557,318 (Buonsante et al), both requiring costly wiring of the building.
However, with regard to known systems utilizing the public subscriber telephone network for communication, there are no systems that also facilitate use of local direct communication circuits to dedicated reply sets (preferably addressed from a common keypad and preferably having a common discriminator/logic circuits), if desired by a tenant. Prior systems require that each tenant has a telephone connected to the public telephone network, in order to facilitate communication with the entrance. With regard to small firms or offices, having one telephone installation only, it is obviously undesirable to block such a telephone by entrance calls or conventional incoming telephone calls, preventing the other type of calls. Furthermore, if one central unit is arranged to serve a number of independent entrance sets, a continuing call via the public subscriber telephone network established from one entrance set will prevent the establishment of other entrance calls via the public telephone network from other entrance sets (provided that the central unit has only one circuit connected to the public telephone network). A direct line connection will not be blocked. Certain tenants may also be opposed to installing a telephone, connected to the public telephone network.